Substrate independent distributed bragg reflector and formation method

ABSTRACT

Distributed Bragg reflectors may be formed in fewer layers by the method, which is capable of producing greater differences in indexes of refraction. Group III-V alternating layers are deposited. The microstructure of alternating layers is controlled to be different. A combination of alternating polycrystalline layers or amorphous and polycrystalline layers results. Alternate ones of the layers oxidize more quickly than the others. A lateral wet oxidation of the alternate ones of the layers produces a structure with large differences in indexes of refraction between adjacent layers. The microstructure between alternating layers may be controlled by controlling Group V overpressure alone or in combination with growth temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIM

[0001] This Application is a divisional application of application Ser.No. 09/635,815, filed Aug. 9, 2000.

STATEMENT OF GOVERNMENT INTEREST

[0002] This invention was made with United States government assistancethrough National Science Foundation (NSF) Grant No. ECD 89-43166 andDARPA Grant No. F49620-98-0496. The government has certain rights inthis invention.

FIELD OF THE INVENTION

[0003] The present invention is in the opto-electronics field.

BACKGROUND OF THE INVENTION

[0004] Distributed Bragg Reflectors (DBRs) are a fundamental componentof optical devices requiring an optical gain, such as various types ofsemiconductor lasers. Conventional vertical DBR's are formed fromlattice-matched alternating semiconductor layered materials. Thesematerials provide a small difference in index of refraction betweenadjacent layers. As a result, a high number of pairs are required in aconventionally formed DBR to obtain desired reflectivities, e.g., about25 to 40 pairs to attain reflectivities as high as 99.9%, depending onthe difference of the index of refraction in adjacent layers.

[0005] Lateral wet oxidation has been used to form a DBR in singlecrystal semiconductor materials. Al-bearing semiconductors are oxidized,and the technique has produced DBRs in, for example, vertical-cavitysurface-emitting lasers. Operation of these types of devices waspossible in certain wavelength ranges. Long wavelength devices were notrealized due to a lack of high quality oxide layer in the laterallyoxidized semiconductor devices. In addition, portions of the visiblespectrum were not supported by the DBR's formed by oxidation due toabsorption of light. These types of DBRs are also limited to latticematched substrates, limiting their application.

[0006] Thus, there is a need for an improved DBR and a method forforming DBRs which addresses the aforementioned drawbacks. The method ofthe invention is directed to this need.

SUMMARY OF THE INVENTION

[0007] In the method of the invention, Group III-V alternating layersare deposited. The microstructure of alternating layers is controlled tobe different during deposit. A combination of alternatingpolycrystalline layers or amorphous and polycrystalline layers results.Alternate ones of the layers oxidize more quickly than the others. Alateral wet oxidation of the alternate ones of the layers produces astructure with large differences in indexes of refraction betweenadjacent layers. The microstructure between alternating layers may becontrolled by controlling Group V overpressure alone or in combinationwith growth temperature and rate.

[0008] The polycrystalline and amorphous materials allow the reflectorto be deposited on any host substrate or device. Changing the thicknessof constituent layers allows creating of reflectors for a wide varietyof wavelengths. Highly reflective DBRs which reflect in the shortwavelength portions of the visible spectrum and deep into theultra-violet wavelengths can be formed by the method. The high-energybandgap materials provide an advantage during processing because oftheir resistance to oxidation. This permits oxidation at highertemperatures, leading to faster oxidation rates and higher throughput.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The invention alters microstructure between formation of adjacentGroup III-V polycrystalline or amorphous layers to create layers thatoxidize at greatly different rates. A lateral wet oxidation thenproduces a significant difference in index of refraction betweenadjacent layers. Highly reflective DBRs formed by the invention may berealized with only a few layers due to the large difference in index ofrefraction.

[0010] DBRs may be formed by any suitable Group III-V formation methodin accordance with the invention. Molecular beam epitaxy (MBE) offerssome advantages. It permits very accurate control of layer thickness. Inaddition, the use of MBE permits formation of DBR material immediatelyfollowing formation of a device structure without breaking vacuum in thesystem used to grow a device including a DBR. However, MBE is notrequired to practice the invention. Exemplary devices discussed in thisapplication were formed by MBE.

[0011] The invention requires formation of alternating Group III-Vlayers of either alternate polycrystalline or alternate polycrystallineand amorphous materials. The microstructure of the alternating layers iscontrolled to produce greatly varying, i.e., by about ten times or more,oxidation rates between adjacent layers. The polycrystalline andamorphous material layers permit greater variance in oxidation ratecompared to the single crystal materials for a comparable oxidationtemperature. This permits a lateral oxidation to create a largedifference in index of refraction between adjacent layers. The change inmicro crystal structure between adjacent layers may be realized bycontrolling Group V overpressure with or independent of growthtemperature.

[0012] DBRs formed by the invention do not require lattice matching tothe underlying material due to the use of an amorphous orpolycrystalline material on the underlying material. Manipulating thegrowth temperature, group V overpressure, and growth rate can accuratelycontrol microstructure of the alternating layers.

[0013] The reflectivity of a DBR is a function of the difference in theindex of refraction between the two adjacent layers and the number ofpairs of these layers. The invention increases the difference in theindex of refraction between adjacent layers due to the oxidation ofalternate layers occurring at a higher rate than intervening layers. Afewer number of pairs are therefore needed to obtain a highreflectivity. An added advantage is a much wider bandwidth of thereflectivity spectrum.

[0014] An exemplary DBR was formed including polycrystalline GaAs layersalternating with amorphous AlAs layers. By depositing the materials atrelatively low growth temperatures of 100° C. at a growth rate of 0.5monolayer (ML)/s, the arsenic overpressure was changed resulting inamorphous compounds for higher overpressures (2×10⁻⁷ torr) andpolycrystalline materials for lower overpressures (8×10⁻⁸ torr). Thematerial was further processed to laterally oxidize the amorphous(Al,As) layers forming an Al-oxide/GaAs DBR. Polycrystalline AlAs layerscan be used for the wet oxidation as well. Oxidation temperatures of300° C. were used to convert the AlAs to its oxide. The Al-oxide has amuch lower index of refraction, around 1.48, as compared to thesemiconductor material that has a refractive index of about 3.36.Polycrystalline GaAs layers withstand higher oxidation temperatures thanthe amorphous (Al,As) layers.

[0015] The ability to control the micro crystal structure to practicethe method of the invention has been demonstrated by adjusting the Asflux and substrate temperature in exemplary devices to deposit GaAs orAlAs with different crystal structures. Specifically, structures can beepitaxial single crystalline on a GaAs substrate if the growthtemperature is high, e.g., 580° C. The structures become polycrystallineif the growth temperature is reduced to below 150° C. In this case, theycan be deposited on any substrates (not necessarily on GaAs substrates).Lowering the substrate temperature even further to below 100° C. makesthe (Ga,As) or (Al,As) become amorphous. The temperatures at whichdeposited layers become polycrystalline or amorphous is also dependenton the amount of As flux. In short, lower temperatures and higher Asfluxes favor the formation of polycrystals to ultimately amorphousmaterials.

[0016] The formation of DBRs using the present method is not restrictedto the materials of GaAs and AlAs. Due to absorption of light at shorterwavelengths, GaAs is not useful for DBR applications in the visiblespectrum. By simply replacing the GaAs layers with GaP, a DBR was formedwhich reflects in the visible spectrum covering the importantred-yellow-green wavelengths. The control of the microstructure of theGaP material was similarly achieved through the manipulation of thegroup V overpressure, in this case phosphorus. While the obviousadvantage of using GaP is the transparency of the material atwavelengths of greater than 550 nm, an additional advantage is thehigher oxidation temperatures allowed. GaP can withstand much higheroxidation temperatures than GaAs allowing the fabrication of thevisible-spectrum DBR at higher oxidation temperatures of 450° C. Higheroxidation temperatures simplify material processing procedures.

[0017] The use of amorphous (Al,As) for the oxide layer is not required.Other Al-bearing compounds, such as amorphous or polycrystallineAlxGalxP, can be used depending on the application. The present methodcan also be applied to the nitride material system using anAl_(x)In_(y)Ga_(1-x-y)N/Al-oxide DBR prepared by a similar procedure.For example, polycrystalline GaN or AlInGaN containing a low compositionof Al can be used as the non-oxidized layer while amorphous orpolycrystalline AlInGaN with a high composition of aluminum can bedeposited and then oxidized for the oxide layer in the DBR structure.This permits fabrication of DBRs in the blue and violet part of thevisible spectrum and possibly ultraviolet applications.

[0018] The significance of making substrate-independent polycrystallineand amorphous materials is that they can be wet oxidized much fasterthan their single crystalline counterparts. For III-V semiconductormaterials, Al-bearing compounds oxidize much faster than non-Al-bearingcompounds. Controlling the crystal structure allows alternating layeredstructures consisting of amorphous/polycrystalline Al-bearing layers andintervening non-Al-bearing polycrystalline layers. Differentialoxidation rates between alternating layers can be enhanced by not onlythe difference in composition (Al-bearing vs. non-Al-bearing) but alsothe difference in structure (amorphous vs. crystalline).

[0019] Wet oxidation of the multi-layer structure produces an efficientDistributed Bragg Reflector (DBR) by converting the Al-bearingsemiconductor layer to Al-bearing oxide layer and leaving thenon-Al-bearing semiconductor layer intact because of the difference inoxidation rate. The large difference in refraction indices between theAl-bearing oxide and non-Al-bearing semiconductor layer makes it abetter DBR requiring fewer pairs to achieve a high reflectivity.

[0020] Other Group III-V materials systems will be apparent to artisans.The method of the invention produces DBRs in which adjacent layers haverelative differences in indices of refraction exceeding 50%. Somealternate combinations are now listed with an indication of thewavelengths reflected. In the structures listed below: “a” indicatesamorphous, “p” indicates polycrystalline, and “n” indicates the index ofrefraction.

[0021] DBR Structure #1

[0022] a-AlAs

[0023] p-GaAs

[0024] n1=3.1/n2=3.6

[0025] After differential oxidation, the a-AlAs becomes a-Al₂O₃ bylosing As resulting in:

[0026] a-Al₂O₃

[0027] p-GaAs

[0028] n1=1.5/n2=3.6

[0029] The DBR won't absorb light whose wavelength is longer than 8800 Åso it is a good DBR design to reflect light longer than 8800 Å, i.e.near infrared and beyond.

[0030] DBR Structure #2

[0031] p-AlAs/p-GaAs nl=3.2/n2=3.4

[0032] After differential oxidation, the p-AlAs becomes a-Al₂O₃ bylosing As.

[0033] a-Al₂O₃/p-GaAs n1=1.6/n2=3.4

[0034] The DBR won't absorb light whose wavelength is longer than 8800 Åso it is a good DBR design to reflect light longer than 8800 Å, i.e.near infrared and beyond.

[0035] DBR Structure #3

[0036] a-AlAs

[0037] p-GaP

[0038] n1=3.1/n2=3.37

[0039] After differential oxidation, the a-AlAs becomes a-Al₂O₃ bylosing As resulting in:

[0040] a-A1₂O₃

[0041] p-GaP

[0042] n1=1.5/n2=3.37

[0043] The DBR won't absorb light whose wavelength is longer than 5500 Åso it is a good DBR design to reflect light longer than 5500 Å, i.e.yellow-green and beyond.

[0044] DBR Structure # 4

[0045] p-AlAs/p-GaP nl=3.2/n2=3.37

[0046] After differential oxidation, the p-AlAs becomes a-Al₂O₃ bylosing As.

[0047] a-Al₂O₃/p-GaP n1=1.6/n2=3.37

[0048] The DBR won't absorb light whose wavelength is longer than 5500 Åso it is a good DBR design to reflect light longer than 5500 Å, i.e.yellow-green and beyond.

[0049] DBR Structure #5

[0050] a-InAlP

[0051] p-GaP

[0052] n1=3.2/n2=3.37

[0053] After differential oxidation, the a-InAlP becomes a-InAlPO₄,resulting in:

[0054] a-InAlPO₄

[0055] p-GaP

[0056] n1=1.6/n2=3.37

[0057] The DBR won't absorb light whose wavelength is longer than 5500 Åso it is a good DBR design to reflect light longer than 5500 Å, i.e.yellow-green and beyond.

[0058] DBR Structure #6

[0059] a-AlP

[0060] p-GaP

[0061] n1=3.0/n2=3.37

[0062] After differential oxidation, the a-AlP becomes a-AlPO₄ resultingin:

[0063] a-AlPO₄

[0064] p-GaP

[0065] n1=1.6/n2=3.37

[0066] The DBR won't absorb light whose wavelength is longer than 5500 Åso it is a good DBR design to reflect light longer than 5500 Å, i.e.yellow-green and beyond.

[0067] DBR Structure #7

[0068] a-Al_(x)Ga_(1-x)P

[0069] p-GaP

[0070] n1=3.3/n2=3.37 (where x>0.9)

[0071] After differential oxidation, the a-Al_(x)Ga_(1-x)P becomesa-Al_(x)Ga_(1-x)PO₄ resulting in:

[0072] a-Al_(x)Ga_(1-x)PO₄

[0073] p-GaP

[0074] n1=1.6/n2=3.37

[0075] The DBR won't absorb light whose wavelength is longer than 5500 Åso it is a good DBR design to reflect light longer than 5500 Å, i.e.yellow-green and beyond.

[0076] DBR Structure #8

[0077] a-Al_(x)Ga_(1-x)P

[0078] p-GaN

[0079] n1=3.3/n2=2.4 (where x>0.9)

[0080] After differential oxidation, the a-Al_(x)Ga_(1-x)P becomesa-Al_(x)Ga_(1-x)PO₄ resulting in

[0081] a-AlxGal-xPO₄

[0082] p-GaN

[0083] n1=1.6/n2=2.4

[0084] The DBR won't absorb light whose wavelength is longer than 3600 Åso it is a good DBR design to reflect light longer than 3600 Å, i.e.near ultraviolet and beyond.

[0085] DBR Structure #9

[0086] a-Al_(x)In_(y)Ga_(1-x-y)N

[0087] p-GaN

[0088] (unmeasured refraction indexes)

[0089] After differential oxidation, the a-Al_(x)In_(y)Ga_(1-x-y)Nbecomes a-Al_(x)In_(y)Ga_(1-x-y)N O_(z) resulting in

[0090] a-Al_(x)In_(y)Ga_(1-x-y)NO_(z)

[0091] p-GaN

[0092] The DBR won't absorb light whose wavelength is longer than 3600 Åso it is a good DBR design to reflect light longer than 3600 Å, i.e.near ultraviolet and beyond.

[0093] In the exemplary DBR structures, an advantage of using phosphiderather than arsenide as the oxide layer is that the phosphide oxide isstronger when the gallium or indium content is higher than about 6-10%.

[0094] While various embodiments of the present invention have beenshown and described, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

[0095] Various features of the invention are set forth in the appendedclaims.

What is claimed is:
 1. A distributed Bragg reflector, comprising: afirst Group III-V layer; and a second Group III-V layer upon said firstGroup III-V layer, wherein a difference in index of refraction betweensaid first Group III-V layer and said second Group Ill-V layer exceeds50%.
 2. The distributed Bragg reflector of claim 1 wherein the secondGroup III-V layer comprises an oxide of a layer configured to oxidizemore quickly than the first Group III-V layer.
 3. The distributed Braggreflector of claim 2 wherein the second Group III-V layer is oxidized bylateral wet oxidation.
 4. The distributed Bragg reflector of claim 2wherein the first Group III-V layer and the second Group III-V layer areformed by molecular beam epitaxy.
 5. A distributed Bragg reflector,comprising: a first Group III-V layer of polycrystalline material; asecond oxidized Group III-V layer upon said layer of polycrystallinematerial, the second oxidized Group III-V layer being one of an Albearing polycrystalline and amorphous material.
 6. The distributed Braggreflector of claim 5 wherein the second oxidized Group III-V layercomprises an oxide of an amorphous (Al, As) layer.
 7. The distributedBragg reflector of claim 5 wherein the second oxidized Group III-V layercomprises an oxide of a polycrystalline AlAs layer.
 8. The distributedBragg reflector of claim 5 wherein the first Group III-V layer comprisesGa and one of As, N, and P.
 9. The distributed Bragg reflector of claim5 wherein the second oxidized Group III-V layer comprises an oxide ofone of amorphous and polycrystalline Al_(x)Ga_(1-x)P (x>0.9).
 10. Thedistributed Bragg reflector of claim 5 wherein the first Group III-Vlayer comprises an Al_(x)In_(y)Ga_(1-x-y)N (x>0.9) layer.
 11. Thedistributed Bragg reflector of claim 10 wherein the second oxidizedGroup III-V layer comprises an oxide of one of amorphous andpolycrystalline AlInGaN.
 12. The distributed Bragg reflector of claim 11wherein an Al composition of the first Group III-V layer is lower thanan Al composition of the second oxidized Group III-V layer.
 13. Adistributed Bragg reflector, comprising: a plurality of alternatinglayers, each adjacent pair of the alternating layers comprising a firstGroup III-V layer including a non-Al-bearing polycrystalline layer and asecond Group III-V layer including one of an amorphous and apolycrystalline Al-bearing oxide layer.
 14. The distributed Braggreflector of claim 13 wherein a difference in indices of refractionbetween the first Group III-V layer and the second Group III-V layer isgreater than 50%.
 15. The distributed Bragg reflector of claim 13wherein the first Group III-V layer comprises Ga and one of As, N, andP.
 16. The distributed Bragg reflector of claim 13 wherein the secondGroup III-V layer comprises one of Al₂O₃, InAlPO₄, Al_(x)Ga_(1-x)PO₄(x>0.9), and Al_(x)In_(y)Ga_(1-x-y)NO_(z) (x>0.9).